In the art of fluid catalytic cracking it is known to crack a gas oil feed in a cracking zone at an elevated temperature in the presence of a cracking catalyst. The cracking reaction tends to deposit coke on the catalyst. Thus, at the conclusion of the cracking reaction the resultant vapor products are separated from the catalyst particles and the latter are regenerated in a regeneration zone by burning coke off of the catalyst. The catalyst is circulated between the cracking zone and the regeneration zone, whereby coke-laden spent catalyst separated from the products is delivered to the regeneration zone for regeneration, and regenerated catalyst, freed of its coke deposit and discharged from the regeneration zone, is returned to the cracking zone for contact with additional gas oil feed. Many suitable types of apparatus for performing such reactions are known to persons skilled in the art. For a number of examples see U.S. Pat. No. 4,200,520, column 1, lines 45-60. See also U.S. Pat. Nos. 4,066,533 and 4,070,159 to George D. Myers, et al.
The gas oil feed conventionally employed in fluid catalytic cracking typically comprise virgin gas oils, recycled streams from fluid catalytic cracking and thermally cracked material boiling below about 1050.degree. F., typically in the range of about 600.degree. F. to about 1050.degree. F., and more typically less than 1025.degree. F. or in many cases less than 1000.degree. F. All of these feeds typically contain rather small amounts of heavy metals such as nickel and vanadium, which are understood to be present in the form of high molecular weight organo metallic compounds. The quantity of such metals present in the feeds may for example be expressed in terms of equivalent nickel, which is the total amount of nickel (Ni) content plus 20% of the vanadium (V) content of the feed, i.e., Ni +V/5.
During the course of the cracking reaction the above-mentioned metals are deposited on the circulating catalyst. During the life of a given catalyst particle, as it repeatedly is exposed to feed, the accumulation of metallic nickel and vanadium thereon progressively increases. Generally speaking the operators of fluid catalytic cracking units practice continuous or intermittent introduction of fresh catalyst to the unit to make up for any losses of catalyst from the system and/or to assist in manintaining the desired level of catalytic activity in the catalyst inventory. As persons skilled in the art will readily appreciate, the nickel and vanadium accumulation may vary from one particle to another within the inventory, but the overall or average inventory of metals on catalyst is in general a function of the amount of such metals present on the fresh catalyst added to the system, the amount of such metals present in the feeds supplied to the system, the relative quantities in which feed and catalyst are brought into contact with one another over an extended period of operation, and the amounts of catalyst introduction and withdrawal (if any) which occur during such operating period. It has been reported that in typical fluid catalytic cracking operations the process is controlled in such a manner as to cause the metal content of the catalysts to equilibriate at a level in the range of about 200 to about 1400 parts per million based on the weight of the catalyst.
Heretofore the present inventor has in certain applications filed in the sole name of the present inventor and jointly with Lloyd E. Busch, described the RCC process, a process of fluid catalytic conversion of carbo-metallic oils to liquid products similar to those obtained in FCC processes. The RCC process is for example disclosed in U.S. Pat. Nos. 4,341,624, 4,347,122, 4,299,687, 4,354,923 and 4,332,673 the entire disclosures of which are hereby incorporated by reference. In RCC processing the feed as a whole will normally contain substantial proportions of components which will not boil below 1025.degree. F. or 1050.degree. F. Such high boiling components generally impart to the carbo-metallic oil feeds considerably higher Conradsen or Ramsbottom carbon values than are typical with gas oil feeds. Moreover these higher boiling components are frequently or usually the repositories of the organo-metallic compounds which are the source of the nickel and vanadium contamination of the feed. Thus in carbo-metallic oil feeds one will generally find a metals content, expressed in equivalent nickel (see above) or nickel equivalents of at least about 4. Nickel equivalents may be expressed in terms of the equation: ##EQU1## wherein each of the above indicated metals is expressed in parts by million by weight of such metal, as metal, based on the weight of the feed. Because of the much larger metals contents of carbo-metallic oils, they tend to impart to the circulating catalyst inventories substantially higher metals accumulations than are found in FCC processing. For example, the metals accumulation on catalyst may range from substantially in excess of 600 ppm nickel equivalents to as high as 70,000 ppm nickel equivalents. In many instances the nickel equivalents of metals accumulated on the catalyst inventory will exceed 3000 ppm. The combined operating difficulties associated with high metals and coke precursors in feed and high rates of metal and coke deposition on catalyst have represented a severe challenge for refiners.
Standing somewhat as a middle ground between these two extremes is the practice of fluid catalytic cracking blends of gas oil, as above described, with various residual oils, the latter containing significantly more metals and/or coke precursors than are common in the typical FCC gas oils. For example see U.S. Pat. Nos. 3,781,197 and 3,785,959 by Millard C. Bryson et al. These patents disclose the cracking of a gas oil admixed with controlled amounts of residual oils, which may or may not have been hydro-desulphurized, over zeolite catalysts. Reportedly, such procedure provides improved yields of gasoline and may improve the clear octane value of certain boiling ranges of the resultant liquid fuel, i.e. gasoline, product. The teachings of this art, in common with prior FCC practice, regard the accumulation of metal on the catalyst as deleterious.